Alternative Seismic Strengthening for Sherith Israel in San Fancisco ...

Alternative Seismic Strengthening forSherith Israel in San Francisco, CaliforniaTERRENCE F. PARETThis paper proposes that engineeringfor preservation be as dominant agoal of the engineering communityas engineering for seismic safety.There is tension in the dialectic betweenengineering and preservation wheneverhistoric buildings undergo the scrutinyof a seismic assessment — tension thatis not prone to being resolved throughcompromise. The engineering community,intent on assuring seismic safety inaccordance with codes for new construction,is, for the most part, disinclinedto analyze archaic structuralsystems for actual performance and isfrequently unwilling to live with anythingless than full compliance withprescriptive code requirements.As an alternative, so-called state-ofthe-artperformance-based standardsmight be offered up — standards such asASCE 31 and ASCE 41 — that are,Fig. 1. South elevation, Sherith Israel, San Francisco, California. David Wakely Photography, 2010.despite representations to the contrary,just as prescriptive and ill-suited forapplication to historic structures as thebuilding codes that govern new construction.1 At the same time, the preservationcommunity, well-intentioned buthobbled by limited expertise in mattersof seismic safety, is reliant on the engineeringcommunity and is, for the mostpart, unable to negotiate from a positionof knowledge. In the end, as preservationgoals become subservient to safetyconcerns, preservationists reluctantlybut silently acquiesce, and nearly allhistorically sensitive obstacles with thepotential to derail the engineering goalsfall. If sufficient funds are found to employtop-end seismic-mitigation strategies,such as base-isolation, the majorityof the original character-defining featuresmay sometimes be saved. However,in the normal course of events, nearly asgreat sums are spent to create a “reenactment”of the original features thatare torn out to make way for a codecompliantseismic-resistant structure.But what if seismic-safety considerationsdid not always necessitate such difficultchoices and impossible compromisesbetween the goals of preservation andseismic safety? What if engineering forpreservation were as dominant a goal ofthe engineering community as engineeringfor seismic safety?Recently, in San Francisco, California,just such a project in which the dialecticdescribed above was radically transformedsaved a masterful and beautifulbuilding, Sherith Israel, from the wreckingball. While various technical aspectsof this project have been described elsewhere,2 the focus of this paper is toarticulate a new philosophy and framework— demonstrably successful in thecase of Sherith Israel — for the seismicassessment and strengthening of a moregeneral class of historic structures.21

22 APT BULLETIN: JOURNAL OF PRESERVATION TECHNOLOGY / 43:2-3, 2012Fig. 2. Sanctuary. All images by WJE taken in2010, unless otherwise noted.BackgroundSherith Israel, a monumental, multistorybrick-masonry synagogue, was designedby Albert Pissis, a prominent San Franciscoarchitect trained at l’École desBeaux-Arts, and constructed in 1904(Fig. 1). The austere, plan-articulatedexterior of the massive, domed buildinghas elements from Classical Revival,Romanesque, and Islamic architecture,typical of synagogue design in theUnited States. The south elevation is themost ornate of the four sides, witharched leaded-glass windows set wellback into the thick masonry walls. Atthe top of the building a slate-roofeddome measuring 60 feet in diameter, itspeak nearly 130 feet from grade, risesabove a fenestrated drum. The building’sornately painted sanctuary, whichhouses 1,400 permanent seats and occupiesthe bulk of the building volume,is one of the last surviving paintedplasterinteriors by turn-of-the-centuryartist Attilio Moretti (Fig. 2). It featuresopalescent stained-glass windows designedby Emile Pissis, brother of thearchitect. The sanctuary also contains afine organ made by Murray M. HarrisCompany, a focal point for assembledcongregants.Sherith Israel survived the great 1906earthquake with relatively minor damage,much of which is still visible, sincethe historic sanctuary finishes have notmodified since the earthquake. It temporarilyserved as home of the CaliforniaSuperior Courts following the collapseof San Francisco City Hall. Thebuilding’s performance during 1906 isbut one testament to Pissis’s abilities asan architect and engineer; he designedmany of the pre-1906 monumentalbuildings extant today in San Francisconeighborhoods that were otherwisedecimated by the earthquake and fire.The synagogue was threatened withclosure due to non-compliance with thelocal Unreinforced Masonry Building(UMB) Ordinance. The ordinance requiredeither demonstrating that thebuilding would protect life safety in theevent of a major earthquake or upgradingthe building if it were determined tobe deficient. 3 The ordinance languageidentified a prescriptive methodology forthe assessment but also permitted use ofthe California Historical Building Code(CHBC). 4 That code, which explicitlyencourages the use of alternative methodsto demonstrate equivalence, ultimatelyprovided the mechanism thatpermitted the building to be saved.While various engineering firms hadbeen engaged over the course of twodecades to assess the building and todevelop solutions that complied with theordinance, all of them opted to employprescriptive approaches in the regularcode rather than the alternative provisionsof the CHBC.Those structural solutions, however,would have resulted in severe damage tothe historic character-defining featuresof the interior and exterior of the buildinghad they been implemented. Oneproposed scheme, for example, recommendeddemolishing much of the finelytooledColusa sandstone exterior andenveloping the building in shotcrete;another would have destroyed largeportions of the interior murals. All thesolutions proposed installation of eitherconcrete-and-metal deck diaphragms orlarge, articulated, structural-steel trussesthat would have destroyed additionaldecorative plaster finishes. None of thesolutions properly credited the existingstructural system for its proven ability toprovide substantial seismic resistance;therefore, all of the solutions requiredthe installation of far greater new, modernstructure than was actually required.Fortunately, the congregation was troubledby the prospect of destroying thebuilding’s beauty and historical significancewhile attempting to save it seismically,and as the mandatory deadlinesbuilt into the ordinance approached, thecongregation sought a new direction andretained another consultant.Goals and Methods OverviewAt the outset of the project, it becameclear that an entirely new approach wasneeded to accommodate the physicaldistinctiveness of the structure. Successfullypreserving the building whileproviding seismic safety would requireovercoming the predisposition of allmodern earthquake engineering methods,even industry-accepted performance-basedprocedures, to penalizestructural systems and configurationsthat do not fit neatly into pre-establishedcategories that are designed primarilyto address the most commonmid-twentieth-century constructiontypes. For example, while the vast majorityof seismic mass of many historicmasonry buildings originates in theexterior walls, the vast majority of theseismic mass of nearly all mid-centurybuildings originates in the floors. Thissingle difference had the potential toresult in vast disparities between thelikely behavior of Sherith Israel and thebehavior anticipated by modern engineeringpractice. During the seismicassessment, it was recognized that thisspecific mass disparity must be accountedfor if the assessment were bothto correctly identify the beneficial attributesof the original structure thatcould be leveraged to improve the overallbuilding response to ground shakingand to pinpoint the critical vulnerabilitiesthat needed to be mitigated. Thus,the overarching goal of the approachfor Sherith Israel was to establish a setof assessment priorities that could beused to fairly assess the seismic adequacyof the historic, non-conformingstructure.Fair assessment was judged to be notachievable using established performance-basedstandards such as ASCE31 and ASCE 41, in part because theywere written for a different and newerinventory of buildings; because they

ALTERNATIVE SEISMIC STRENGTHENING 23penalize the structural particulars ofmost historic buildings; and becausethey can be expected to fail all buildingsthat do not exhibit the structural characteristicsand configuration that areencouraged by the codes and standardsthat govern new design. The frameworkdeveloped included the following criteria,which were implemented in both aphilosophical and a practical sense.These criteria are generally applicable tothe seismic assessment of any historicstructure.Create and enforce a hierarchy ofequals between seismic or structuraland preservation goals. This hierarchyis envisioned as the embodiment of thenotion of “engineering for preservation.”Neither seismic safety nor preservationtook precedence in the SherithIsrael project; both were worthy goalsthat were pursued simultaneously andwith equal intensity. Of note, on complexseismic-assessment and strengtheningprojects for historic buildings, onesure way to begin the project on thewrong track is to parse these dual goalsbetween the disparate disciplines ofengineering and architecture, as is normallydone. These dual goals cannot beproperly pursued in isolation by designprofessionals whose views of the builtenvironment are fundamentally notreconcilable and who see fundamentallydifferent things when they examine ahistoric building. Therefore, on theSherith Israel project, these dual goalswere treated as if they were one, andboth goals were paramount from theinception of our involvement. At everyturn, the focus was on trying to findstructural means for achieving seismicsafety with less historic impact, includingcrediting the existing structure tothe greatest extent justifiable, modifyingand/or relocating structural interventionsto minimize impact, and studiouslyconsidering the requirements ofinstallation.Seek to preserve historic structuralsystem as much as preserving finishes.Historic structure, the bones ofthe building, as opposed to its finishes,is not only a valuable historic asset inits own right; it is also usually valuableas a structural asset. The existing structurelikely already provides substantialseismic resistance and should not bedismissed out of hand as noncompliant.Though the structure may not incorporatemodern-day precepts of how abuilding should be constructed to resistearthquake ground shaking, the structurecertainly has inherent strengthsthat can be leveraged and employed toachieve equivalence or near-equivalenceto modern-day accepted levels of seismicsafety. Whether the historic structuralcomponents are hidden behindfinishes or not, their preservation is aworthy goal, without which structuralinterventions will necessarily be moredamaging than they would otherwiseneed to be. Moreover, at Sherith Israelpreservation of the historic structure asa contributor to seismic resistance, notas a non-participatory museum piece,was a primary goal. Supplementation ofthe existing structure was preferred oversupplantation. The tendency of theengineering profession to view historicstructures as a quaint but incompetentand unreliable component of a rehabilitatedstructure only encourages thedestruction of what could be preserved.Implement “beyond-code” thinking toavoid destroying a building’s historicvalue while attempting to save it. Theprescriptive provisions in the InternationalBuilding Code (IBC), which hasbeen by now adopted nearly everywherein the U.S., were developed under thepremise that the buildings to which theyare to be applied have not yet beenconstructed and are only in the initialstages of conception. 5 Just as thispremise is wholly inapplicable to historicbuildings, the provisions in theIBC are wholly unsuitable to applicationto historic buildings. Forcing ahistoric structure into a set of guidelinesdesigned for new construction is thequickest way to undermine preservationgoals.In the last decade in the U.S., theInternational Existing Building Code(IEBC) has been developed and sporadicallyadopted. 6 Although this code isspecifically intended to regulate workthat is being done on existing buildings,it is just as prescriptive as the IBC and isnot appropriately responsive to theneeds of historic buildings. Fundamentally,its goal appears to be to requirethat existing buildings ultimately getstructurally strengthened to satisfy theFig. 3. Sherith Israel, 1906. Courtesy of BancroftLibrary, University of California, Berkeley.lateral force requirements of the IBC.Though its preamble language statesotherwise, it allows none of the flexibilitythat is required to properly assess orimprove non-conforming structuralsystems. The reader is strongly encouragedto look to resources other thanInternational Code Council (ICC) documentswhen addressing historic buildingsand to avoid referencing thesedocuments during the initial stages of aproject, when all project participantswill get locked into them as projectcriteria. The use of alternative analysisprocedures that make sense for thestructure being evaluated, similar tothose set forth in the CHBC, is stronglyrecommended. If appropriate publishedprocedures applicable in the jurisdictiondo not exist, then the engineering teammay have to justify their assessment andstrengthening recommendation usingfirst principles. In contrast to prescriptiveICC documents, ASCE 31 andASCE 41, the CHBC provided the flexibilityneeded to address the special considerationsrequired to simultaneouslypreserve and seismically strengthenSherith Israel. 7Resist the temptation to turn thebuilding into one that performs like amodern structure. Independent of thedegree of seismic safety they afford,historic structures will necessarily behavedifferently in an earthquake thannew, code-conforming structures, due toinherent differences in constructionmaterials, construction technology,configuration, massing, etc. There issimply no way to make an older structurebehave like a modern one duringstrong ground shaking, unless the pre-

24 APT BULLETIN: JOURNAL OF PRESERVATION TECHNOLOGY / 43:2-3, 2012Fig. 4. Plaster cracking in organ machine room,adjacent to north exterior wall, caused by the1906 earthquake.existing structural system is replaced bya new one. These differences in responseshould be embraced to the greatestdegree justifiable; they are not necessarilyproblems that need to be overcome.Properly evaluated, many historicbuildings will be found to require littleintervention relative to that involved inreplacement, because they already havestructural systems with substantialability to resist lateral forces.By embracing the differences andleveraging the capacity that the existingsystem provides, the extent of disruptionnecessary to achieve the required level ofseismic safety can often be minimized.Unfortunately, whether by misunderstandingor misapplication of the codesand municipal requirements, relativedifficulty of proper analysis, concernsabout liability, or a desire to make asmall engineering involvement larger, theengineering community is highly motivatedto severely discount the seismiccapacity of existing non-conformingstructural systems and thereby create animplicit need to replace old with new. Atmany times during the course of a seismicproject involving historic structures— and most often in its early stages,when criteria are being developed andagreed upon — there will be decisionson the table where the temptation tomake the building behave like a modernone can greatly ratchet up the eventualdisruption to character-defining features.The initial stages of the project are thecritical time to set expectations as to theproject goals.Key Aspects of the ApproachThe approach described below wasdeveloped to minimize the potential forconflicts between engineering goals andpreservation goals at Sherith Israel andwas a natural outgrowth of the criteriadescribed above, tailored to the specificsof its structural type, its documentedhistory, the specific requirements of theSan Francisco Unreinforced MasonryBuilding Ordinance, and the concernsof the congregation. In general, thisapproach is applicable to the seismicassessment and strengthening of anyhistoric structure.Identify the positive attributes of theexisting structural system and develop“surgical” approaches to leveragethem. The Sherith Israel projectwas successful because of the deep focuson developing an understanding of andappreciation for the structural systemalready in place, rather than yearningfor a structural system that did notexist. Every historic structural systemhas one or more competent structuralfeatures with inherent positive attributesthat are potentially capable ofproviding a framework for conceptualizing,designing, and then building acomplete seismically resistant system.Therefore, initial steps in any historicbuildingseismic-assessment and seismic-strengtheningproject ought to includedefining these features and attributesand outlining what beneficialbehavior they already contribute toseismic safety.These features should not be viewedas something that must either be overcomeor removed, simply because theydo not comply with current prescriptivecode provisions for new construction.Rather, the goal of this process is toinitiate the identification of the existingstructural features that are the bestpotential candidates for leveraging, thustaking a first step down the path towarddevelopment of targeted engineeringmethods to leverage them, which bothprovides an engineering-based reasonfor retaining and re-using the existingsystem and sets the groundwork forminimizing disruption to the building asa whole. The core competence of thefeatures identified during this processshould be viewed as the best availablefoundation for satisfying the project’sseismic-safety goals while utilizing theexisting historic structure to the greatestextent possible — the essence of “engineeringfor preservation.”The basic steps used for SherithIsrael, which are possible approaches forother historic buildings, are as follows:Employ the historical record. The historicalrecord for Sherith Israel provideda significant amount of informationthat was used to provide perspectiveon the seismic characteristics of thebuilding. In addition to the few architecturaldrawings retained by the congregation,the archives of local historicalsocieties and universities and ofSherith Israel yielded the following: aneyewitness account of the 1906 earthquakedamage to neighboring buildings,including important information on theseverity of shaking; an eyewitness accountof the exterior damage to SherithIsrael, which provided solid informationon the behavior of the exterior masonrywalls during the earthquake; receiptsrelated to earthquake-damage repair;and a photograph of the building exteriorduring repairs (Fig. 3). 8 This infor-Fig. 5. Inclined step-cracking and 2-inch offset inmasonry gable-end wall.

ALTERNATIVE SEISMIC STRENGTHENING 25Fig. 6. Screen shots of SAP 2000 analyses, which display greatly magnified representations of deformationsas the building sways.mation proved invaluable in benchmarkingstudies to assess the seismicresistance of the building.Employ physical research. The best wayto develop an understanding of a building’sattributes is through on-site detectivework, especially when the buildingof interest has undergone few modernizations.Hands-on physical study ofSherith Israel, initiated long beforemore formal engineering studies, revealed1906 earthquake damagethroughout the building, particularly inplaster finishes and in the masonrygable-end walls (Figs. 4 and 5). Thephysical research yielded clear, factbaseddata on the manner in whichSherith Israel moved during the shaking,offering both a glimpse of its likelyresponse mode in a future large earthquakeand a baseline against which toconduct benchmarking studies.Conduct engineering simulations ofdisplacement. Computer-basedquantitative simulations of buildingdisplacement are relied upon heavily inearthquake engineering assessments.For Sherith Israel a suite of analyseswas designed to explore multiple aspectsof the behavior of the structureand of its primary components. Dynamicanalyses using SAP 2000 wereconducted, in part to define the shapesthat the building structure takes onduring swaying (Fig. 6). 9 Nonlinearanalyses of the masonry walls subjectedto in-plane and out-of-plane excitationpermitted the simulation of the damageto the walls that would occur during a“design earthquake.” These studies canbe used with great effect, particularlyfor identifying the specific areas mostprone to experiencing safety-compromisingdamage, thus enabling the focusof interventions to be narrowed to areasthat actually require mitigation. For themasonry walls in Sherith Israel, state-ofthe-artadaptive pushover techniquesusing ADINA, a general-purpose finiteelementanalysis program well-suited tononlinear analysis, were used (Figs. 7and 8). 10 By application of these analyticalmethods, the predicted behavior ofthe masonry walls in a future earthquakewas benchmarked by comparingthe explicit predictions of cracking inthe masonry that were generated by theanalyses to the cracking observed duringsurveys of the masonry walls in thebuilding (Figs. 9 and 10). Such verification,conducted during a series ofbenchmarking studies, is critical to theprocess of developing a sound understandingof the expected strengths andweaknesses of the structural system.Although studies of this type werecomplex, time-consuming, and expensive,they were more than justified bythe reduction in the degree of interventionrequired, reducing project costs byan even larger factor.Employ engineering artfully. As a generalrule, the best engineering is thatwhich accomplishes the most with theleast materials, elegantly. The corollaryto this for historic structures is that thebest engineering accomplishes the mostwith the least disruption, elegantly: itaccomplishes the greatest degree ofpreservation by continuing to the greatestdegree possible the useful functionof that which already exists. Brute-forceengineering is neither artful, elegant,nor desirable. Of course, the fact thatevery historic structural system hassome positive attributes does not necessarilymean that those attributes aresufficient in and of themselves to form astructural system that provides resistanceto earthquake loading adequate tosatisfy the project’s safety or performancecriteria. What it does suggest,however, is that by careful identificationof positive attributes and by carefulselection of intervention methods, theirattributes can be leveraged to accreteadditional seismic benefit.The key to this step is in seekingtargeted, low-impact or disruption-freemethods for accomplishing the leveraging;this approach requires balancing thebenefit gained with the disruption requiredto accomplish it. This exercisedoes not seek to merely identify the leastFig. 7. Screenshot of nonlinear pushover analysisdepicting out-of-plane deformation of southexterior wall.

26 APT BULLETIN: JOURNAL OF PRESERVATION TECHNOLOGY / 43:2-3, 2012Fig. 8. Stress plots derived from nonlinear analysis of the south exterior wall, interior face (left) andexterior face (right). The highly stressed area in the right panel is in the attic at an articulation in thesouth wall, produced by a simulation of the 1906 earthquake as the wall tried to “unfold” during outof-planeresponse.material that must be added; it is criticalthat the engineer also explicitly considerthe means of access that is necessary forthe contractor to add the structureenvisioned. It may be that the existingstructure can be made seismically adequateby, for example, adding a single,inexpensive bolt at each existing connection,but if the disruption caused byaccessing each connection is great, thenthe result is not desirable, and the projectwill be a failure, no matter theelegance of the single-bolt solution.introduce sufficient risk such that theyclearly require mitigation or, at least,management.Elimination of all seismic risk cannotbe put forth as a reasonable goal for anybuilding, even those that are completelycompliant with every prescriptive provisionin the IBC. Although in the designof new construction the engineer maynot expressly conduct any relative rankingof the seismic risk that ensues fromhis design, especially since such anexercise would fall outside the statedrequirements of the building code, seismicrisk associated with the product ofthe code-design process still exists, andan engineer so inclined could carry outsuch a ranking exercise. However, forhistoric buildings there is a real need toidentify and prioritize the extant risks,not merely because this provides a rationalmeans for gaining insight into thelikely performance of the building duringan earthquake, but also because it isa necessary step toward meaningfulincorporation of an “engineering forpreservation” philosophy into the projectmethodology. The attitude thatseismic risk must be eliminated wholesalewill only lead to unreasonabledemolition and removal of the historicfeatures of the building that are thetargets of the preservation project; thereis no reason that historic buildingsshould be held to seismic safety standardsthat are greater than for newconstruction.Like the process of identifying thepositive attributes of the historic structurethat can be best leveraged, segregationof the critical vulnerabilities fromthe others is a major step down the pathtoward development of engineeringmethods to mitigate them “surgically.”Such a distinction provides both anengineering-based reason for retainingand re-using as much of the existingsystem as possible and sets the ground-Identify the critical vulnerabilities ofthe existing structural system anddevelop “surgical” approaches tomitigate them. The Sherith Israel projectwas successful because of the focuson the need to place its potential vulnerabilitiesinto a meaningful and practicalcontext in lieu of seeking to eliminateevery possible vulnerability. Everyhistoric building has numerous vulnerabilitiesthat are potentially capable ofintroducing risk to life safety in theevent of an earthquake, but not everyvulnerability introduces the same levelof risk. Initial steps in any seismicassessmentand strengthening projectfor a historic building therefore oughtto include defining the potential vulnerabilitiesand attempting to conceptuallyisolate the critical ones, i.e., those thatFig. 9. The heavy black lines indicate the predicted crack patterns on interior face of west exteriorwall.

ALTERNATIVE SEISMIC STRENGTHENING 27Fig. 10. Wide stepped cracking, believed tohave formed during the 1906 earthquake,appears in the south wall in the attic at a majorplan articulation. The crack was strong-backedby bolted steel angles at an unknown date.work for minimizing disruption to thebuilding as a whole.The steps used for Sherith Israel,which are possible approaches for otherhistoric buildings, are those that weredescribed above. The documented historicalrecord, hands-on physical examinationof the building, and engineeringsimulations of displacement, both forthe purpose of developing insights intothe conceptual behavior of Sherith Israellikely to occur during strong groundshaking and to explicitly conduct benchmarkingstudies, were all extremelyvaluable exercises. No published methodologycould possibly have providedthe level of validation achieved by thesecomplementary studies. Having a firmerunderstanding of which portions of thestructure and finishes presented thegreatest risks enabled the project to focuson mitigation of the risks that matteredmost, thereby eliminating wastefuland time-consuming assessment anddesign time, as well as wasteful andexpensive disruption to the buildingitself.The Solution for Sherith IsraelThe seismic-strengthening plan forSherith Israel was developed to fullyleverage the inherent capacity of theunreinforced masonry structure, as wellas satisfy the intent of and the provisionsin the CHBC that permitted utilizationof analysis and design methodologiesalternative to those in the regularcode. The plan was permitted in2009, and the first phase of the constructionhas been completed. Thescheme leaves untouched essentiallyevery historically significant interiorfinish in the sanctuary and nearly allhistorically significant finishes elsewherein the building — a degree of preservationthat would have been impossible toachieve had traditional seismic-strengtheningmethods been employed.In addition to heavy reliance on thestudy of the physical evidence of 1906earthquake damage and historical documentsdescribing damage in San Franciscoin general and to Sherith Israel inparticular, development of the planrelied on modern-day estimates of theintensity of shaking during the 1906event 11 ; on a variety of analytical structuralstudies of the building, includingstate-of-the-art adaptive pushover techniques;and on laboratory testing ofstate-of-the-practice structural materialsthat would be incorporated into thedesign. The structural solution not onlypreserves the historic fabric of SherithIsrael but also preserves and fortifies thepositive structural attributes that enabledthis building to survive the 1906earthquake while nearby masonry structureswere damaged beyond repair.Specifically, the solution takes advantageof the dynamic separation betweenLoad (lbs)600050004000300020001000Nitinol Wire4.0% Elongationthe modes predominated by in-planeand out-of-plane wall shaking, an attributethat is normally eliminated whenfloor and roof diaphragms are stiffenedin the usual course of seismic strengthening.The solution employs a combinationof established and innovative interventions,each developed to minimizedisturbance to the nonstructural finishesand retain the structure’s original dynamiccharacteristics while improving itsintegrity. These interventions includecenter-cored reinforcement of the masonrywalls 12 ; a system of tension ties inthe attic that interconnect the four perimeterwalls, yet circumvent the domedsanctuary; and nonlinear, compressiononlypilasters and fiber-reinforcedpolymer, in addition to more typicaltechnologies such as bond beams andfloor-to-wall ties. Every system andcomponent of the design was expresslydeveloped with a close eye on the degreeof disruption required to install it.Center cores consist of shafts drilledwithin the plane of the masonry walls,both horizontal and vertical, into whichreinforcing DYWIDAG threaded rodsand grout are placed. The center cores inSherith Israel were designed primarily as“integrity steel,” which would crosspotential cracks as predicted by thenonlinear analyses and restrain thegrowth of those cracks, thus limiting thepotential for large pieces of masonry00.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50Elongation (percent)4.0% 45F 4.0% 90F 4.0% 72FFig. 11. Laboratory test result of nitinol wire cycled in tension between zero load and 4 percentelongation at three different temperatures.

28 APT BULLETIN: JOURNAL OF PRESERVATION TECHNOLOGY / 43:2-3, 2012falling out of the walls during an earthquakeand compromising the gravitycarryingcapacity of the otherwiseunreinforcedbearing walls. Althoughthe City of San Francisco prohibits theuse of center cores to boost shearstrength of unreinforced masonry, thecenter cores were permitted to be usedto make wall segments with high heightto-thicknessratios acceptable. Installingreinforcing in the walls also improvestheir toughness.Most of the center-core shafts weredrilled from the roof and into the foundation,typically 70 feet or more inlength, within the thickness of a fourwythewall. The reinforcing placed inthe 3-inch- and 4-inch-diameter drilledshafts in Sherith Israel was encased in apolymer grout, custom designed andoptimized via laboratory testing. Polymergrout was selected primarily becauseit would eliminate the possibilityof damaging historic plasters that wouldexist with any water-based grout andbecause its structural properties can bemore compatible with historic masonrythan cementitious grouts. Withoutspecial care, however, polymer groutscan introduce problems that outweighthe advantages of using them. The groutfor Sherith Israel was optimized tominimize bleeding, shrinkage, crackingdue to the thermal change associatedwith curing, and cost and to improveconsolidation.The reinforcing that was droppedinto the center-core holes projectedabove the top of the holes and wasterminated into the reinforced-concretebond beam along the top of the wall.The reinforced-concrete bond beamreplaced the original interior sandstonecoping that had been roofed over andhidden.Tension ties in monumental houses ofworship are by no means innovative;they have been relied upon for millenniato supplement the out-of-plane stabilityof masonry bearing walls. The tensionties in Sherith Israel, however, rely onsuper-elastic nitinol wires, designed tobe lightweight and easy to install, torestrain the walls from falling outwardand to provide a “self-centering” action,while maintaining the inherent flexibilityof the system that enabled the structureto survive the 1906 earthquake. Asdemonstrated by laboratory testingconducted in support of the mitigationtechnology designed for Sherith Israel,nitinol wire can be “stretched” to about105% of its length and fully recover itsoriginal length when the load on it isreleased (Fig. 11). Nitinol is used mostcommonly in medical and optical applicationsbut only rarely in structuralones. Although it has been extensivelystudied for seismic purposes in universitylaboratories, to date nitinol isknown to have been used in seismicapplications only in Italy to improve theseismic response of medieval cathedralsdamaged during the Assisi earthquake in1997. 13 Use of nitinol in Sherith Israel isbelieved to be the first application forimproving seismic resistance in the U.S.and to be the first application of thistype (employing wires in direct tensionwithout need of machining) anywhere.The nonlinear, compression-onlypilasters were employed selectively toimprove the performance of SherithIsrael’s north wall, which was the onlyexterior wall of the structure withoutsignificant plan-articulation; it was alsolacking adjacent horizontal diaphragmsegments equivalent to the other exteriorwalls. Without the stabilizing pilastersthat originate from the “folding” of thewall in plan and significant supportingdiaphragms, the north wall was determinedto be more vulnerable that theothers. The absence of these articulationsand floor diaphragms was accommodatedto some degree in Pissis’s designby locating the stairwells and thefour plaster-sheathed walls that boundthese stairs against the north wall. However,movement of the wall during 1906caused the most significant cracking inthe plaster in the building.Because the north wall is along analleyway and is effectively shielded fromview from the street, external pilasterswere designed to supplement its out-ofplanestability, thereby avoiding disruptivework in the interior of the building.The compression-only pilasters supportthe wall to prevent it from falling awayfrom the building toward the north.However, by the incorporation of specialreleases that allow the pilasters to uplift,they permit the wall to freely displacesouthward when the building as a wholesways toward the south. This detail wasconsidered to be necessary to preventthe new pilasters from tearing the buildingapart when forces induce displacementtoward the south.ConclusionA new approach and philosophy for theseismic assessment and strengthening ofhistoric buildings has been described, aswell as a methodology for its implementation.The approach was recently implementedon Sherith Israel, a monumental,unreinforced-masonry synagoguein San Francisco, with a domed,mural-painted sanctuary and an austerebut magnificent sandstone exterior. Theapproach resulted in a seismic-strengtheningscheme that literally saved thebuilding from the wrecking ball, sinceprior solutions were both too disruptiveto Sherith Israel’s character-definingfeatures and too expensive to implement.TERRENCE F. PARET, a senior principal atWiss, Janney, Elstner Associates, Inc. (WJE),has performed hundreds of engineering investigations,many involving the seismic evaluationof structures before and after earthquakes andthe design of earthquake-damage repairs andseismic rehabilitation. He is currently managingWJE’s seismic assessment of the WashingtonMonument. He can be reached at tparet@wje.com.Notes1. American Society of Civil Engineers, SeismicRehabilitation of Existing Buildings ASCE/SEI31-03 (Reston, Va.: ASCE, 2003). AmericanSociety of Civil Engineers, Seismic Rehabilitationof Existing Buildings ASCE/SEI 41-06,(Reston, Va.: ASCE, 2007). ASCE 31 and ASCE41 are problematic for all existing structures.For suggested reading on the general subject ofthe problems inherent in these documents, seethe following: Gary R. Searer, Terrence F. Paret,and Sigmund A. Freeman, “ASCE 31 and ASCE41: What Good Are They?” in Proceedingsfrom the ASCE Structures Congress, Vancouver,Canada, April 2008, and Terrence F. Paret,Gary R. Searer, and Sigmund A. Freeman,“ASCE 31 and 41: Apocalypse Now,” in ASCEStructures Congress, Las Vegas, Nevada, April2011, proceedings on CD.2. Terrence F. Paret, Sigmund A. Freeman, GaryR. Searer, Mahmoud M. Hachem, and Una M.Gilmartin (2006), “Seismic Evaluation andStrengthening of an Historic Synagogue UsingTraditional and Innovative Methods and Materials,”in Proceedings of the First EuropeanConference on Earthquake Engineering andSeismology, Geneva, Switzerland, September 3-8, 2006, and Sigmund A. Freeman, Terrence F.Paret, Gary R. Searer, Mahmoud M. Hachem,and Una M. Gilmartin, “Using Historical Datato Aid in the Evaluation of Structural Performanceof Buildings,” in Ninth Canadian Conferenceon Earthquake Engineering, Toronto,

ALTERNATIVE SEISMIC STRENGTHENING 29Canada, June 2007, proceedings on CD. TerrenceF. Paret, Sigmund A. Freeman, Gary R.Searer, Mahmoud M. Hachem, and Una M.Gilmartin, “Using Traditional and InnovativeApproaches in the Seismic Evaluation andStrengthening of a Historic UnreinforcedMasonry Synagogue,” Journal of EngineeringStructures 30, no. 8 (Aug. 2008): 2114–2126.Mahmoud M. Hachem, Terrence F. Paret, GaryR. Searer, and Sigmund A. Freeman, “TheEvaluation and Retrofit of a Historic UnreinforcedMasonry Building Using NonlinearAdaptive Pushover and Dynamic AnalysisMethods,” Paper Number 05-04-0110, in 14thWorld Conference on Earthquake Engineering,Beijing, China, October 12-17, 2008, proceedingson CD. Terrence F. Paret, Daniel H. Eilbeck,Sigmund A. Freeman, and Gary R. Searer,“Seismic Assessment and Strengthening of HistoricUnreinforced Stone Masonry Structures:Technical and Philosophical Considerations,” inInternational Scientific Committee on the Analysisand Restoration of Structures of ArchitecturalHeritage (ISCARSAH), Mostar, Bosnia,July 12, 2009, proceedings on CD, pp. 101–108.3. “Earthquake Hazard Reduction in UnreinforcedMasonry Bearing Wall Buildings,”Chapter 16B, 2001 San Francisco BuildingCode, City and County of San Francisco, 2001.4. California Historical Building Code, CaliforniaCode of Regulations, Title 24, Part 8.5. International Code Council, Inc., InternationalBuilding Code (Washington, D.C.: ICC,2009).6. International Code Council, Inc., InternationalExisting Building Code (Washington,D.C.: ICC, 2009).7. A comparison between these documents isbeyond the scope of this paper. What can bepositively asserted is that all ICC documents,the IBC and the IEBC included, are prescriptiveby design and intent, and they are traditionallyenforced prescriptively by building departmentsthroughout the U.S. In most editions of theIEBC, when seismic work is triggered, whetherby alterations or repair, the entire building hasbeen required to conform to current IBC requirements.However, IEBC provisions forhistoric buildings are rapidly evolving. IEBCChapter 11 has long required that for historicbuildings, a report be prepared that itemizes thecontributing features of the building that wouldbe damaged by compliance with the structuralrequirements of other chapters, items not incompliance, and how compliance with theintent of the provisions is to be achieved. It alsorequires that in higher seismic-design categories,a structural evaluation be performed that describesthe load path and earthquake-resistantfeatures. Although this is, perhaps, a step in theright direction, nothing in these editions fulfillsthe “motherhood” language of the IEBC, whichstates that its intent is to provide flexibility topermit the use of alternative approaches. Untilthe 2012 edition, the IEBC set forth no substantiveexceptions for historic buildings with respectto the prescriptive structural requirementsor to any of the upgrading triggers, which areonerous from the standpoint of preservation. Inthe 2012 edition, however, substantial changeswere made that appear to provide greaterlatitude for historic structures, particularly withrespect to repair. However, as with much newcode language, there are inconsistencies andambiguities, particularly with respect to alterations,that need to be resolved before the realimpact of these new provisions can be determined.The ASCE documents referred to are standards;they are neither prescriptive codes norperformance codes. Although they pertain toperformance-based engineering and set forthperformance criteria, they are highly prescriptivein that they dictate how to achieve thestated performance levels and provide virtuallyno flexibility unless the engineer is willing toignore or modify the published provisions, assuggested by other standards. References areprovided in endnote 3 that discuss some of themany problems with these documents.The CHBC was specifically designed andwritten to promote performance-based assessmentsand rehabilitation and to allow the engineerto use discretion rather than dictating howto do assessments prescriptively. The CHBCcontains no triggers that lead to building-widestructural strengthening. In addition to the performancecriteria that it sets forth, it contains asingle prescriptive chapter that may be invokedif seismic strengthening is triggered by someother action, such as a local ordinance. Everyyear, however, strong pressure is exerted bysome members of the engineering profession toturn this last true performance-based documentinto another prescriptive code. These effortsreinforce the dire need for the philosophy ofengineering for preservation to be forcefullypromoted by the preservation community.The San Francisco UMB Ordinance is aprescriptive portion of Chapter 16 of the SanFrancisco Building Code that was adopted inresponse to a statewide requirement that unreinforcedmasonry buildings be seismically evaluated.The ordinance is highly prescriptive;however, it permits historic buildings to beaddressed using the CHBC.8. Photograph and eyewitness account are in thecollection of the Bancroft Library, University ofCalifornia, Berkeley, Calif.9. Computer and Structures, Inc., SAP 2000version 9.1 (2004), Berkeley, Calif.10. ADINA R&D Inc. (2005), “ADINA:Theory and Modeling Guide,” Report ARD05–7, Watertown, Mass. Stress plots and crackplots from nonlinear analyses are designed toconsolidate and quickly communicate volumesof information about structural behavior on asingle sheet, but they must be viewed with apracticed eye. The multi-wythe masonry wallsof Sherith Israel were necessarily idealized ascontinuous homogenous systems and thendiscretized, through their thickness and acrosstheir length and height, meaning that localeffects due to variable construction quality, forexample, cannot be accurately captured. On theother hand, plotted values are output elementby element or node by node, meaning that theplots have the appearance of being highlyaccurate at the local level. In reality, analyses ofthis type are most reliable for identification oflarger-scale behavior. One such behavior revealedby the Sherith Israel studies was the tendencyfor the plan-articulated walls to “unfold”during out-of-plane response. This predictedbehavior causes cracking at or near the articulations,starting at the top of the walls, as wasidentified in the building in all three of thehighly articulated walls.11. J. Boatwright, email communication, 2005.12. David C. Breiholz, “CenterCore StrengtheningSystem for Seismic Hazard Reduction ofUnreinforced Masonry Bearing Wall Buildings,”Structure (May 2003): 18–20.13. L. Janke, C. Czaderski, M. Motavalli, and J.Ruth, “Applications of Shape Memory Alloys inCivil Engineering Structures: Overview, Limitsand New Ideas,” Materials and Structures 38(June 2005): 578–592.The APT Bulletin is published by theAssociation of Preservation TechnologyInternational, an interdisciplinaryorganization dedicated to the practicalapplication of the principles andtechniques necessary for the care and wise use ofthe built environment. A subscription to theBulletin and free online access to past articles aremember benefits. For more information, visitwww.apti.org.